The present invention relates to sensor network systems capable of continuously tracking a situational change in the state, position or the like of people or an object, and in particular relates to the fields in which locationing of a mobile object is effective.
Conventionally, in the fields of: the security management with respect to behavior of people in buildings, urban areas and the like; the commodities management in distribution process in warehouses, stores and the like; the health and safety management of people in medical facilities or homes; and the condition monitoring of pets, livestock and the like, there has been proposed and also been put in practical use a method in which a tag provided with a means for transmitting an ID capable of identifying the individual to the outside is attached to mobile objects such as people, commodities, animals or the like, and then the information on the tag is read from the outside, thereby managing the mobile objects.
In managing a mobile object, one of the important information to be managed is the position of the mobile object in addition to the identification of the individual by means of an ID. By combining the ID and the position of a mobile object with a time when it is observed, it is possible to obtain information, such as the current position of a specific mobile object, a route of movement, relation between a plurality of mobile objects, relation with an observed field, and the like. In the above-described fields, for example, a situation of breaking into a restricted entry area by a facility worker, tracing of a distribution channel of commodities, checking of a patient's location, and the like can be grasped from the above information.
As a method for identifying the position of a mobile object, there is disclosed a method for using a wireless terminal such as a mobile phone, as a tag for sending an ID, and a base station for communicating with the wireless terminal. For example, there is a method in which wireless communication base stations are arranged such that the communication areas may not overlap to each other, and when a wireless terminal communicates with the nearest base station, the position of this wireless terminal is assumed to be the position of the base station to which this communication is carried out.
There is also a method, in which a means for measuring time until a signal of a wireless terminal reaches a base station is provided, and at least three or more base stations receive the signal wave from the wireless terminal simultaneously, and based on the measuring results of the differences in time when the radio wave reached, the distance between each base station and the terminal is estimated and thus the position is identified based on the principle of trilateration (see JP-A-08-129061 for example).
There is also a method in which in place of the time difference, the distance between each base station and a terminal is estimated from the difference in the radio field strength of signals received from the terminal to thereby identify the position based on the principle of trilateration (see JP-A-11-178042 for example).
Moreover, as a method for identifying the position of a wireless terminal based on the principle of trilateration, there is a method in which a statistical model of the radio field strength is determined by a training for each target space, and then by using this model the measurement error is reduced (see JP-A-2004-112482 and JP-A-2005-525003 for example).